I submitted the following feature request to the request bord:

"Instead of having the functionality of the WeightPaint
mode under edit buttons (vertex group selection and
weight) and the paint buttons (opacity), I think it
would make more sense to join these functions together
under the paint buttons. The weight numberbutton could
be changed into two seperate buttons with title "Add"
(weight=1.000) and "Sub" (weight=0.000) so you don't
have to type the numbers everytime you want to change
from adding weight to decreasing weight or vice versa.

To improving the workflow even more, the functionality
of these two buttons could be used while actually
painting the weights: painting with left mousebutton
pressed (as it is handled now) would be adding weight
and painting with the left mousebutton AND a key
pressed (i.e. SHIFT or CTRL) would substract weight.
Something similar to the masking tool in Gimp."

What do you think?